1. Field of the Invention
The present invention relates to a powered seat slide device, such as a motor-driven seat slide device for an automotive vehicle, and specifically to a powered seat slide device employing at least one pair of guide rails, namely an upper slidable guide rail firmly secured to the bottom surface of the automobile seat and a lower stationary guide rail rigidly mounted on the floor panel of the vehicle body, which is capable of electrically adjusting the position of the automobile seat assembly in a direction generally parallel to the longitudinal direction of the vehicle body.
2. Description of the Prior Art
A conventional automotive powered seat slide device employs a telescopically extendable guide rail structure for each side of the seat assembly. The telescopically extendable guide rail structure is usually interposed between the seat cushion and the floor panel of the vehicle.
Referring now to FIGS. 1 and 2, there is shown a conventional powered seat slide device for automotive vehicles. The telescopically extendable guide rail structure is comprised of an upper slidable guide rail 1 firmly secured to a respective side portion of the base plate of the seat and a lower stationary guide rail rigidly mounted on the floor panel. The powered seat slide device also includes a guide rail driving mechanism being comprised of a drive motor 2, such as a DC motor, and a gear box 8 including a worm 9 having a driven connection with a drive shaft 2b of the motor and a worm wheel having external threads meshed with the worm 9. As seen in FIG. 2, the drive motor 2 is firmly secured to the side wall section 1a of the upper slidable guide rail 1 by means of fasteners, for example a fastening bolt 4 utilized with a well-type self-locking nut 3, a board anchor, a blind rivet or the like. Reference numeral 5 designates a circular opening 5 for directly fitting the nut 3. The gear box 8 serves as a reduction gear for reducing revolution of the output shaft 2b of the motor 2 and increasing rotation torque created therethrough. The worm wheel is rigidly formed integral with a screw-threaded shaft 7 rotatably supported by the upper guide rail 1 through bearings at both ends thereof. Reference numeral 6 is a cut-out 6 formed at the inner side wall section 1a of the upper slidable guide rail 1, for introducing a cylindrical hub section formed integral with the a main motor casing 2a of the motor 2 thereinto. The output shaft 2b of the motor is rotatably supported by the above noted cylindrical hub section of the motor and outwardly extended The output shaft 2b has a driving connection with an input shaft 9a of the worm 9. Generally, the output shaft 2b is directly connected to the input shaft 9a as shown in FIG. 2. Alternatively, the output shaft 2b could be connected through a rigid coupling to the input shaft 9a, as disclosed in Japanese Patent First Publication Tokkai (Showa) 63-34252. In order to insure smooth torque transmission from the output shaft 2b of the motor to the input shaft 9a of the worm, the two shafts 2b and 9a are precisely aligned with each other.
In the previously noted conventional powered seat slide devices, the two shafts 2b and 9a can be precisely aligned with each other only under a particular condition wherein the plane defined by the ceiling wall section 1b of the upper slidable guide rail 1 is precisely formed perpendicularly to the plane defined by the side wall section 1a through a precise machining, such as a bending process with an extremely high machining accuracy. Supposing that the machining accuracy in the bending process is less than a predetermined criterion, the angle sandwiched between the side wall section 1a and the ceiling wall section 1b is offset from a right angle (90.degree.). Thus, the two shafts 9a and 2b could be misaligned with each other. As a result, smooth and high torque transmission is prevented and consequently noise occurs at the connecting portion between the two shafts 9a and 2b, when the machining accuracy in the upper guide rail 1 is low. Furthermore, since the drive motor 2 is directly connected to the side wall section 1a of the upper guide rail 1 having a relatively low rigidity, the guide rail 1 experiences resonance through the direct connection with the upper guide rail 1, because the guide rail 1 must receive reaction force created due to moment of inertia of the rotor enclosed in the motor 2, during rotation of the motor. Under this condition, a noise level generated during operation of the driving mechanism is increased with great energy loss and as a result an exciting current value of the motor 2 is also increased, since the drive motor is susceptible to such resonance. As appreciated from the above, the motor employed in the conventional powered seat slide device has an increased tendency to become overloaded due to the above resonance created between the motor 2 and the upper guide rail 1 or the driving mechanism including the screw-threaded member 7 and the gear box 8. This results in a relatively low durability of the device. Furthermore, since the main motor casing 2a is directly connected to the side wall section 1a of the upper guide rail 1, but not to gear box 8, the alignment matching between the two shafts 2b and 9a is determined only by an installation accuracy in a relative position of the main motor casing 2a to the upper guide rail 1. Assuming that the installation accuracy is low, the previously noted resonance may occur between the motor and the driving mechanism, thereby resulting in a low rotation torque transmission efficiency.